With the advent of the energy crisis there has been substantial emphasis placed on the utilization of the so-called "non-exhaustible" sources of energy such as solar heat. One of the principal problems associated with effective utilization of solar heat has been the cost of storing significant quantities of such heat for use during non-daylight hours or during extended periods when the sun was obscured by cloudy or overcast skies. Indeed, the high cost of construction of storage systems has minimized the effective utilization of solar heat. In general, large volumes of storage media are absolutely essential. At the present time, the cost of providing storage devices of adequate size has proved to be a limiting factor in the utilization of solar heat for other than small scale domestic or residential space and water heating applications.
A typical residential system for the utilization of solar energy is shown in U.S. Pat. No. 4,029,082. The system comprises a storage bin containing heat storage material and a means for channeling air in sequence from a space to be warmed through the storage bin to pick up heat at a low temperature. The warm air is returned to the space to be warmed. Patentees further suggest the addition of water to the air and the storage bin to act as an additional source of heat storage as well as providing humidity to the air.
U.S. Pat. No. 3,989,927 describes another heat storage system for use in space heating which comprises a container having walls formed of a heat resistant material and including a guide duct in the form of a tube extending through the container for carrying a gaseous heat extraction medium. The container also contains heat storage medium comprising heavy metal bearing minerals in the form of a pourable bulk of particulate solids suitable for directly contacting with air to be heated for places of human habitation.
U.S. Pat. No. 4,024,910 discloses a system for the storage of heat or cold to be utilized in maintaining a desired temperature in habital areas. The system utilizes a plurality of channels dug directly into the earth and filled with rocks which are used to absorb heat or cold for storage; a part of the storage being the rocks themselves and usually a much larger part being the adjacent earth. Suitable conduit connections are provided for carrying heat or cold to the rocks.
The foregoing systems, while of interest for residential space heating or even space heating of commercial buildings, do not address the problem faced by a utility industry where a high temperature heat source is required. Thus, others have developed systems in an attempt to provide a heat storage system which would be suitable for use, for example, by a commercial utility power plant.
U.S. Pat. No. 2,933,885 discloses a heat storage accumulator system which comprises a closed loop circuit containing a heat transmitting fluid and including an isolated heat accumulator for the circuit. The heat accumulator contains a heat absorbent medium having a melting point between the low and high temperature limits of the fluid in the closed loop circuit.
U.S. Pat. No. 3,178,113 discloses a heat storage system particularly adapted to space installations. The system is made up of two interconnecting systems, one of which constitutes an energy absorber and heat storage and release unit, and the other comprises a conversion system by which the heat is converted to electricity. The collection and storage system includes a heat energy collector or convertor for collecting heat from the rays of the sun which are reflected onto a boiler. The boiler is in a circuit which includes a heat storage device. The system also includes appropriate valves and controllers for maintaining a substantially constant, desired temperature in the circuit.
U.S. Pat. No. 2,593,963 relates to a binary cycle power plant and utilizes a high melting point tertiary fluid for indirect heating. The proposed system, however, does not provide for the storage of heat, though it does suggest the use of a liquid alkali metal as a fluid heat transfer medium.
Other patents considered during the preparation of this application but not considered sufficiently pertinent to warrant further discussion are U.S. Pat. Nos. 2,680,437; 3,983,929; 3,957,030; 3,968,653; 3,977,197; 3,993,041; and 4,038,555.